Local Area Networks (“LAN”) operate using a variety of data link layer protocols. As is known in the art, the Open Systems Interconnection (“OSI”) model is used to describe computer networks. The OSI model consists of seven layers including, from lowest-to-highest, a physical, data link, network, transport, session, application and presentation layer. The physical layer transmits bits over a communication link. The data link layer establishes and maintains point-to-point connections for the transmission of data frames.
A widely used scheme for managing a LAN is Carrier Sense Multiple Access with Collision Detection (“CSMA/CD”). CSMA/CD is organized around layered protocols that include a data link layer with a Logical Link Control (“LLC”) sub-layer and a Medium Access Control (“MAC”) sub-layer. The LLC sub-layer handles logical links between stations while the MAC controls access to a transmission medium via a physical layer. A CSMA/CD standard for Ethernet LANs is defined by Institute of Electrical and Electronic Engineers (“IEEE”) 802.3 standard.
The CSMA/CD structure is a peer-to-peer network structure and stations on the network are vying for use of a channel when they have data to transmit. The contention can result in collisions that distort the transmission signals. Distorted collision signals cannot be used by stations on the network. A “collision window” is typically used to handle CSMA/CD collisions.
A collision window describes the length of time required for a signal to propagate through the channel and to be detected by each station in the network.
Token bus LANs use a bus topology and provide access to a channel as if it were a ring. The token bus protocol is a collision-free protocol that uses a special frame called a “token” to govern which network station is allowed to send data. Capacity can be allocated through the use of priority classes. IEEE 802.4 defines a token bus LAN standard.
Token ring LANs connect participating stations in a ring topology; each station is connected to medium via a ring interface unit. The ring interface unit is responsible for monitoring data passing through it as well as regenerating a signal and passing it to the next station. Token ring LANs use a token to determine which station may transmit, and also include a priority scheme. The object of the priority scheme is to give each station an opportunity to reserve the use of the ring for the next transmission around the ring. IEEE 802.5 defines a token ring LAN standard.
The CSMA/CD Ethernet LANs, token bus LANs and token ring LANs and other 802.x standards use LLC and MAC in the data link layer. However, there are several problems associated with using CSMA/CD Ethernet LANs, token bus LANs and token ring LANs that use LLC and MAC in their data link layer.
CSMA/CD protocols with LLC and MAC work reasonably well with large numbers of stations, but do not easily or efficiently support multiple priority classes or priority schemes. Token bus and token ring protocols with LLC and MAC are typically more complex than CSMA/CD protocols. Token bus and ring related protocols with LLC and MAC in their data link layer can support multiple priority classes or priority schemes but have poor latency characteristics as the number of network stations grow.
It is desirable, especially in the area of real-time services (e.g., teleconferencing), to have a MAC layer protocol which is simple, efficient, fair and distributed that can be used on a variety of computer network types (e.g., CSMA/CD, token bus and token ring). The MAC layer protocol should allow for a large number of network stations with differing priority class levels and delay requirements.